(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices and more specifically to a method of forming a relaxed underlying layer used to accommodate an overlying stained silicon layer.
(2) Description of Prior Art
The ability to form devices such as a metal oxide semiconductor field effect transistor (MOSFET) in a silicon layer under tensile strain, has allowed the performance of the MOSFET to be increased via enhanced mobility of carriers in the strained silicon channel region. Methods of forming tensile strained silicon layers include forming this layer on an underlying relaxed layer such as a silicon-germanium layer. Relaxed silicon-germanium on an underlying silicon substrate has been called a silicon-germanium virtual substrate. The growth of a relaxed semiconductor layer such as silicon-germanium can be challenging since it encompasses controlled nucleation, propagation, and interaction of misfit dislocations that terminate with threading arms that extend to the surface and then can be replicated in subsequently grown layers such as the overlying strained silicon layer needed for accommodation of a subsequent device. The defects in the strained silicon layer resulting from the misfit dislocations in the underlying relaxed silicon-germanium layer, can deleterious influence MOSFET leakage and yield.
The crystalline quality of the relaxed silicon-germanium layer can be improved by growing a compositionally graded, thick silicon-germanium layer, at a thickness greater than a micrometer. The compositionally graded relaxed layer features decreasing germanium content from the bottom to the top surface of the layer. This solution however is costly in terms of processing cost and time since between about 1 to 3 micrometers of a relaxed layer is needed. In addition the density of the threading arms is not homogeneous in the relaxed silicon-germanium layer, therefore sometimes resulting in regions of overlying strained silicon layers with a higher than desired propagation of defects.
The present invention will describe a method of forming a buffer layer comprised of silicon-germanium, via use of inserting silicon-germanium-carbon layers into a composite layer that terminates with an overlying relaxed silicon-germanium buffer layer. The silicon-germanium layer, overlying a silicon-germanium-carbon layer now features a lower density of threading arms or dislocations than counterpart silicon-germanium layers formed without being a component in a composite layer comprised with a silicon-germanium-carbon component. The silicon-germanium-carbon components of the composite layer act as a defect filtering layer allowing the top layer of silicon-germanium to be a relaxed, low defect density buffer layer for an overlying strained silicon layer. Prior art such as Kanzawa et al in U.S. Pat. No. 6,645,836, Canaperi et al in U.S. Pat. No. 6,524,935, Brunner et al in U.S. Pat. No. 6,403,975, Chu et al in U.S. Pat. No. 6,350,993, and Chu et al in U.S. Pat. No. 6,251,751, describe methods of growing silicon-germanium-carbon, silicon-germanium and silicon-carbon; growing strained silicon on relaxed silicon-germanium; growing epitaxial germanium on strained silicon-germanium; and growing constant or graded silicon-germanium. However none of the above prior art describe the procedure described in the present invention in which all components of the composite layer are grown at the same temperature in situ the same growth system, thus allowing multiple composite layers comprised of silicon-germanium and silicon-germanium-carbon, to easily be obtained. These advantages are expected to improve the degree of strain relaxation as a result of the presence of multiple silicon-germanium-carbon filtering components, as well reducing cost and throughput as a result of using a single growth system and a similar growth temperature for each component of the composite layer.